The sail of a boat sailing at a angle of incidence to the true wind develops lift and drag forces by reason of the flow of air about the airfoil shape of the sail. The air flow is such that low pressure is established along the leeward (convex) side of the sail and high pressure along its windward (concave) side, which, as now generally propounded by theorists in the field, produces circulation of air about the sail, or a bound vortex, which results in the lift and drag forces. The total lift is represented as a lift vector that is perpendicular to the direction of the relative wind and the drag as a drag vector that is parallel to the direction of the relative wind. The total aerodynamic force is the resultant, acting at the center of pressure of the sail, of the lift and drag forces and can be resolved into a thrust vector that is parallel to the heading of the boat and propels the boat forward and a heeling force that is perpendicular to the heading of the boat that is responsible for heeling the boat to leeward. The underwater foils of the boat, either a keel or centerboard and the rudder, develop equal and opposite hydrodynamic forces since they act at an angle of incidence to the flow of water about their surfaces.
As noted above, the aerodynamic force developed by a sail is a result of the pressure differential between the windward surface of the sail (high pressure) and the leeward surface of the sail (low pressure). The pressure differences vary along the sail foil, being greatest along the forward section of the sail. The effectiveness of a sail is primarily dependent on the character and quality of the air flow over its leeward surface, especially along its forward portion. The air flow is laminar over the forward part of the leeward surface, but then changes to turbulent flow and separates from the sail along a transition zone. The location of the transition zone varies with the angle of incidence between the sail and the wind, and is generally thought to be in the range from about 1/10 to 1/3 of the foil length behind the leading edge of the sail. The effectiveness of the sail is decreased when the air flow separates from the sail, and the lift generated by the sail can be increased if the separation can be delayed or moved aft along the sail.
One of the techniques taught in the art for delaying air flow separation is the use of vortex generators to develop tip vortices that act to drag in air from the free air stream beyond the very thin boundary layer next to the surface of the sail to energize the air in the boundary layer so as to delay separation and improve the lift-to-drag ration of the aerodynamic forces. Various constructions for vortex generators have been suggested or proposed, as described later in this specification. The principal objective of the present invention is to provide a simple, economical construction for vortex generators which, it is believed, will enable substantially more widespread use of the device by sailors.